Stabilized levitation of magnetic elements



FLFUL-l-L May 19, 1970 6&4.

G. G. NORTH 3,512,852

STABILIZED LEVI'IATION OF MAGNETIC ELEMENTS Filed March 7, 1969 2Sheets-Sheet l 3/ SENSOR g 38 4/ 33 U BIPOLAR J39 CONTROLLED QDIFFERENCE v0 LTAG E /36 AMPLIFIER SUPPLY m m 37 SENSOR INVENTOR. GEORGE6. NORTH BY mag-4....

ATTORNEY y 1970 G. G. NORTH 3,512,852

STABILIZED LEVITATION OF MAGNETIC ELEMENTS I Filed March 7, 1969 2Sheets-Sheet 2 a /$2. 26b S) dz;

GEORGE 6. NORTH INVENTOR.

ATTORNEY Int. Cl. 30810 United States Patent 12,852 STABILIZEDLEVITATION 0F MAGNETIC ELEMENTS George G. North, Santa Ana, Calif.,assignor to the United States of America as represented by the UnitedStates Atomic Energy Commission Filed Mar. 7, 1969, Ser. No. 805,161Int. Cl. F16c 39/06 Claims ABSTRACT OF THE DISCLOSURE A system forstabilizing a magnetic element, such as as a magnetized disc, conductivering having a circulatory current therein, or the like, levitated in amagnetic field, against movement out of a region of free suspensiontherein. The magnetic element is positioned with its magnetic field inopposition to a vertically oriented magnetic levitation field such thatthe fields interact to freely support the element. against the forces ofgravity. Any tendency of the element to slip laterally from its stablesupport position to unstable support positions is continuously sensedand responsively compensated by the generation of a magneticcompensating field which is effective to restore the element to, andthereby maintain the-element in, its stable support position.

BACKGROUND OF THE INVENTION This invention was evolved under, or in thecourse of Contract W-7405-eng-48 with the United States Atomic EnergyCommission.

Under various circumstances it is desirable to levitate a magneticelement, such as a magnetized disc or current carrying conductive ring,in a magnetic levitation field. A non-materially supported magnetizeddisc freely suspended in space is, of course, advantageously employableas a platform for supporting objects out of thermal, electrical, andfrictional contact with adjacent or surrounding material surfaces.Likewise in various controlled thermonuclear reaction research devicesit is necessary to freely suspend a current carrying conductive ringwithin an ionized plasma magnetically confined within a toroidalchamber. In particular, free suspension of a superconducting closedloop, having a large order circulatory current induced therein, within aconfined plasma permits build-up of a hot ion plasma.

It will be appreciated that in the case of both the magnetized disc andcurrent carrying ring types of magnetic elements, the associatedmagnetic fields are similar. One face of the element is magnetized withnorth polarity while the opposite face is magnetized with southpolarity, and the lines of fiux are directed from the north pole face tothe south pole face in symmetrically disposed reentrant loops extendingabout the periphery of the disc parallel to its axis. Such an element isconsequently levitated when it is disposed with its associated magneticfield in opposition to a vertically oriented uniform magnetic levitationfield, as may be generated, for example, by spaced-apart, verticallyaligned, oppositely polarized magnetic pole pieces. In this regard themagnetic element is disposed in the levitation field with the north andsouth faces of the element respectively facing the north and south polepieces. By virtue of the opposed fields, magnetic forces are generatedwhich offset the downwardly directed gravitational force acting on theelement. The element assumes an equilibrium position of free suspensionin the vertical direction. However, a condition of severe instabilityexists in the lateral retention of the element in suspended position.The interacting magnetic fields have associated forces that are such asto cause 3,512,852 Patented May 19, 1970 ice the element to sliplaterally from a substantially centered position in the levitatingfield, and to invert itself, th reby terminating levitation. Thus, inthe absence of means for overcoming the lateral instability in thesupport of the element, levitation thereof is short lived.

SUMMARY OF THE INVENTION The present invention relates to the stabilizedlevitation of a magnetic element of the previously described type in amagnetic levitation field in order to provide steady-state, non-materialsuspension of the element.

"In the accomplishment of the foregoing, the invention is arranged tocontinuously restore the magnetic element to a stable substantiallycentered position in the levitating field in response to any tendency ofthe element to slip laterally therefrom. More particularly, theinvention includes means for sensing lateral displacements of' themagnetic element from centered position in the magnetic levitationfield, and means responsive to the sensed displacements for generating acompensating magnetic field peripherally about the element having adirectio'ri and magnitude to adjust the configuration of the field ofthe element and the levitation field in a manner productive of forceopposing the displacements to thereby restore the element to itscentered position.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic representation ofa system for the stabilized levitation of magnetic elements inaccordance with the present invention.

FIGS. 2-4 are graphical illustrations depicting the manner in which thesystem of the present invention is effective to stabilize the levitationof a magnetic element.

FIG. 5 is a block diagram of one of the servos employed in the system ofFIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, thereis shown a system in accordance with the present invention forstabilizing the levitation of a magnetized disc 11, or equivalentmagnetic element, such as a current carrying conductive ring, in auniform vertically oriented magnetic levitation field generated, forexample, by spaced-apart vertically aligned, oppositely polarizedmagnetic pole pieces 12 and 13. The opposite faces of the disc areoppositely magnetically polarized such that the disc has an associatedmagnetic field with magnetic fiux directed from the north pole face tothe south pole face in symmetrically disposed reentrant loops extendingabout the periphery of the disc parallel to the axis thereof. Thelevitation field is defined by uniformly distributed axially symmetriclines of magnetic flux directed vertically from the north pole piece 12to the south pole piece 13. The disk is coaxially disposed between thepole pieces with the field of the disc opposing the levitation field,i.e., with the north and south faces of the disc respectively facing thenorth and south pole pieces. The fields interact and establish repulsivevertical magnetic forces which offset the downwardly directedgravitational force acting on the disc. The disc is thus freelysuspended in the vertical direction by the cancellation of forces.

With the disc precisely centered on the pole piece axis, the compositemagnetic field due to the interacting disc field and levitation fieldhas lines of flux B with a configuration substantially as depicted inFIG. 2. In this regard, the flux lines are axially symmetric and bulgeoutwardly about the disc periphery. It will be appreciated that withsuch a symmetric field configuration, the lateral, as well as thevertical components of magnetic force are balanced such that the disc isin a position of stable support. Howeven, any departure of the disc fromprecisely centered position distorts the symmetry of the outwardlybulging portion of the flux lines. There is an attendant unbalance ofthe lateral components of force acting on the disc such that it slips inthe lateral direction away from centered position. Thereafter the discis inverted, thereby terminating levitation.

It will be appreciated that the field configuration depicted in FIG. 2capable of stably supporting the disc represents a singular idealizedcondition requiring precise centering of the disc in the levitationfield and precise uniformity of the disc field and levitation field.Such a singular condition is virtually impossible to obtain and maintainin actual practice by virtue of material irnperfections in the disc andpole pieces, field perturbations, and the like. Consequently it may besaid that support of the dics, while being stable in the verticaldirection, is unstable in the lateral direction. There is a severetendency for the disc to slip laterally in the levitation field awayfrom a centered position of totally stable support such that levitationof the disc is extremely short lived.

To overcome the foregoing problem, the stabilizing system of the presentinvention basically includes position sensing means arranged to sensedepartures of the disc 11 from a laterally centered position coaxialiybetween the pole pieces 12 and 13, and ,to signal the direction andextent of such departures. The sensed position signals are employed toactuate magnetic compensating field generating means operable to adjustthe configuration of the composite disc and levitation field in a mannerproductive of magnetic force opposing the sensed departures andrestoring the disc to centered position. More particularly, the positionsensing means are preferably provided as a plurality of light sources14, 16, 17, and 18 disposed in vertically spaced relation to one face ofthe disc to beam light towards a plurality of photocells, or equivalentlight sensors 19, 21, 22, and 23 disposed in vertically spaced relationto the opposite face of the disc in corresponding alignment with thelight sources. The light sources and sensors are equally radially spacedoutwardly from the axis of the pole pieces at 90 circumferentiallyspaced intervals adjacent the periphery of the disc.

Thus, when the disc is in its centered position diametrically opposedsensors 19 and 21 are exposed to equal amounts of light from theirassociated sources 14 and 16, while diametrically opposed sensors 22 and23 are exposed to equal amounts of light from their associated sources17 and 18. However, when the disc slips laterally from its centeredposition, a sensor of at least one of the diametrically opposed pairsthereof is exposed to more light from its associated source than thelight the other sensor of the pair is exposed to from its associatedsource. This is by virtue of the disc masking less light from the formersensor and more light from the latter sensor by moving away from, andtowards the respective light beams directed thereon. It will be thusappreciated that the direction and magnitude of lateral displacement ofthe disc is indicated by the extents to which the respective sensors areexposed to their associated light sources. Inasmuch as the sensorsgenerate electrical signals in proportion to the amount of lightincident thereon, the signals are representative of disc position. Moreparticularly, the algebraic difference between the signals generated bythe sensors of each diametrically opposed pair thereof is indicative ofthe direction and magnitude of the component of disc movement fromcentered position along that particular diameter. Thus, when the disc isin centered position such that opposed sensors 19 and 21 are exposed tothe same amounts of light and opposed sensors 22 and 23 are exposed tothe same amounts of light, the algebraic difierences between theresulting signals are zero. However, if, for example, the disc movesfrom right to left, as viewed in FIG. 1, along a diameter aligned withopposed pair of sensors 19 and 21, sensor 19 is exposed to more lightthan sensor 21. The algebraic difference between the sensor signals isthen positive and of a magnitude proportional to the extent of discdisplacement from centered position. Conversely, if the disc moves fromleft to right, sensor 21 is exposed to more light than sensor 19. Thealgebraic difierence between the sensor signals is then negative and ofa magnitude proportional to the extent of disc displacement fromcentered position. Similarly, the polarity of the algebraic differencebetween the signals from opposed sensors 22 and 23 indicates movement ofthe disc into or out of the plane of FIG. 1, while the magnitudeindicates the extent of such movement. Combinations of the differencesbetween the signals from the respective diametrically opposed pairs ofsensors indicate lateral movement of the disc in directions other thanthose in alignment with the opposed pairs of sensors.

Considering now the compensating field generating means in detail andthe manner in which the sensor signals are employed to control same, itis to be noted that the compensating field generating means preferablyinclude a pair of opposed arcuate conductors 24 and 26, each extendingsubstantially 180, coaxially disposed with respect to pole pieces 12 and13 in outwardly spaced circumscribing relation to disc 11. Also includedis a second pair of opposed arcuate conductors 27 and 28, each extendingsubstantially 180, coaxially disposed with respect to the pole pieces inoutwardly spaced circumscribing relation to the disc andcircumferentially displaced from the conductors 24 and 26 by Upon thefiow of current through the conductors, magnetic flux is generatedconcentrically thereabout. The sense and magnitude of the compensatingflux is determined by the direction and magnitude of current flowthrough the conductors. The compensating flux interacts with thecomposite d' and levitating field and alters the configuration the operipherally of the disc in accordance with the se se and magnitude ofsuch flux. Thus, the sense and gnitude of the compensating flux may becontrolled to p uce magnetic forces effective to return the disc tocentered position whereupon field symmetry is restored.

In the accomplishment of the foregoing, the signals from the opposedpair of sensors 19 and 21 and pair of sensors 22 and 23 indicating thedirection and extent of lateral displacements of the disc 11 fromcentered position are employed to control the direction and magnitude ofcurrent flow in the opposed pair of conductors 24 and 26, and opposedpair of conductors 27 and 28, respectively to generate compensating fiuxin a manner to overcome the sensed disc displacements. Moreparticularly, a servo 29 is provided with input terminals 31 and 32respectively connected to sensors 19 and 21, a first pair of controlledvoltage supply terminals 33 and 34 respectively connected to theopposite ends of conductor 24, and a second pair of controlled voltagesupply terminals 36 and 37 respectively connected to the opposite endsof conductor 26. Similarly, there is provided a second zero 29' havinginput terminals 31 and 32' respectively connected to sensors 22 and 23,a first pair of controlled voltage supply terminals 33' and 34'respectively connected to the opposite ends of conductor 27, and asecond pair of controlled voltage supply terminals 36 and 37'respectively connected to the opposite ends of conductor 28. The servo29 is arranged such that in response to the signal at terminal 31 fromsensor 19 being greater than the signal at terminal 32 from sensor 21,voltages are generated at supply terminals 33 and 34 and at supplyterminals 36 and 37 with polarities and magnitudes to drive currentsthrough conductors 24 and 26 having appropriate directions andmagnitudes to establish magnetic compensating fiux with the proper senseand magnitude to laterally move the disc towards the sensor 19 supplyingthe greatest signal. Conversely, in response to the signal at terminal32 from sensor 21 being greater than the signal at terminal 31 fromsensor 19, the polarities of the voltages generated at supply terminals33 and 34 and at supply terminals 36 and 37 are reversed and themagnitudes are appropriate to effect lateral movement of the disc towardsensor 21 to centered position. Similarly, the servo 29' is effective tocontrol the directions and magnitudes of current flow through conductors27 and 28 in accordance with the signals from sensors 22 and 23 toestablish the proper sense and magnitude of compensating fiux tolaterally move the disc to centered position toward the sensorgenerating the greatest signal.

' To the foregoing ends, the servo 29 is advantageously provided asillustrated in FIG. 5, and it is to be understood that the servo 29 isprovided in a similar manner. Servo 29 preferably includes a bipolardifference amplifier 38, the differential input terminals of whichcorrespond to terminals 31 and 32 and are thus respectively connected tosensors 19 and 21. The amplifier functions to produce at an outputterminal 39 thereof, a bipolar signal proportional to the algebraicdifference between the sensor signals applied to terminals 31 and 32.For example, in the illustrated case a positive signal having amagnitude proportional to the difference between the sensor signals isproduced at terminal 39 in response to the signal from sensor 19 beinggreater than the signal from sensor 21, whereas a negative signal havinga magnitude proportional to the difference between the sensor signals isproduced at terminal 39 in response to the signal from sensor 21 beinggreater than the signal from sensor 19. When both sensor signals areequal, the signal at terminal 39 is zero.

The output terminal 39 of the difference amplifier is coupled to acontrolled voltage supply 41 having two sets of supply terminalsrespectively corresponding to terminals 33 and 34 and to terminals 36and 37. The supply 41 functions to generate voltages at the outputterminals having polarities and magnitudes in accordance with thealgebraic difference signal applied thereto from the differenceamplifier 38. In addition, the supply is arranged such that the voltagesat terminals 33 and 36 and at terminals 34 and 37, respectivelyconnected to adjacent ends of the conductors 24 and 26, aresimultaneously of the same polarities. In the illustrated case, apositive difference signal is productive of proportional supply voltageswith positive polarities at terminals 33 and 36 with respect toterminals 34 and 37. A negative difference signal is productive ofproportional supply voltages of reversed polarities, i.e., negativepolarity voltages at terminals 33 and 36 with respect to terminals 34and 37. Servo 29 operates in a similar manner such that in response tothe signal from sensor 22 being greater than that from sensor 23,voltages proportional to the difference therebetween are produced withpositive polarities at terminals 33' and 36 with respect to terminals34' and 37'. When the signal from sensor 23 is greater than that fromthe sensor 22, voltages proportional to the difference therebetween areproduced with negative polarities at terminals 33 and 36 withrespect toterminals 34' and 37'.

Considering now the overall operation of the levitation stabilizingsystem, assume that the disc 11 is in its centered position of coaxialalignment with the pole pieces 12 and 13, as shown in FIG. 2. Aspreviously noted, the flux lines B of the resulting composite field areaxially symmetric and bulge outwardly about the disc periphery when thesingular condition of both vertically and laterally stable disc supportexists. The sensors 19 and 21 are exposed to equal intensities of lightfrom sources 14 and 16, and no signals are applied from the sensors toservo 29. Thus, no current flows through conductors 24 and 26 and nocompensating flux is therebv generated.

If the disc slips laterally to the left from centered position, as shownin FIG. 3, sensor 19 is exposed to more light from source 14 while lessli ht from source 16 is received by sensor 21. By virtue of thedifference between the signals applied to the input terminals of servo29, the

servo effects proportional counterclockwise current flow throughconductor 24 and proportional clockwise current flow through conductor26, as depicted by the xs in the figure. Clockwise compensating flux Bis thus generated concentrically about the conductors. It is to be notedthat the compensating flux B being directed clockwise, interacts withthe composite field B in such a manner as to increase the density offlux lines on the left periphery of the disc and reduce the density offlux lines on the right periphery thereof. The attendant magnetic forcesare thus such as to urge the disc towards the right to centeredposition. As the disc masks more and more of sensor 19 from thelightbeam directed from source 14, and exposes sensor 21 to more and more ofthe light beam directed from source 16, the compensating flux B iscorrespondingly reduced and is terminated when the disc reaches itscentered position.

Conversely, if the disc slips laterally to the right from centeredposition, as shown in FIG. 4, sensor 21 is exposed to more of the lightbeam from source 16 while less of'the light beam from source 14 isexposed to sensor 19. The difference between the signals applied to theinput terminals of servo 29 is now such that the servo effectsproportional clockwise current flow through conductor 24 and.proportional counterclockwise current flow through conductor 26, asdepicted by the dots in the figure. As a result, counterclockwisecompensating flux B is generated concentrically about the conductors andinteracts with the composite field B to increase the density of fluxlines on the right periphery of the disc and decrease the density offlux lines on the left periphery thereof. Thus, the magnetic forcesdeveloped are such as to urge the disc towards the left to centeredposition.

In a similar manner, the sensors 22 and 23, servo 29', and conductors 27and 28 operate to stabilize the disc against movement laterally into andout of the plane of FIGS. 2-4. Simultaneous operation of the servos 2'9and 29 may, of course, also occur to effect current flow in both sets ofconductors 24, 26 and 27, 28 having appropriate magnitudes anddirections to stabilize the discs against movement in directions otherthan directly between the opposed sets of sensors. Stabilization of thelateral position of the disc is thereby continuously effected andlevitation thereof is preserved.

I claim:

1. A stabilized magnetic levitation system comprising means forgenerating a uniform axially symmetric vertically oriented magneticlevitation field, a magnetic element generating a magnetic field withfiux directed in symmetrically disposed reentrant loops extending aboutthe periphery of the element parallel to the axis thereof, said elementcoaxially disposed in a centered position within said levitation fieldwith the field of the element opposing said levitation field to therebylevitate said element therein, position sensing means for sensinglateral departures of said element from said centered position andresponsively generating signals representative of the direction andextent of said departures, and means coupled to said position sensingmeans in receiving relation to said signals for responsively generatingmagnetic compensating flux peripherally of said element productive offorces in opposition and proportional to the direction and extent ofsaid departures represented by said signals, whereby said element iscontinuously restored to said centered position.

2. A stabilized magnetic levitation system according to claim 1, furtherdefined by said position sensnig means comprising first and seconddiametrically opposed pairs of light sources disposed in verticallyspaced relation to a first face of said element to beam light adjacentthe periphery of said element, said second pair of light sourcescircumferentially spaced from said first pair of light sources, andfirst and second diametrically opposed pairs of light sensors disposedin vertically spaced relation to a second face of said element oppositesaid first face in corresponding vertical alignment with said first andsecond pairs of light sources to receive the light beamed therefrom, andsensors generating electrical signals proportional to the light receivedfrom said sources, where-by the algebraic differences respectivelybetween the signals from said first pair of sensors and the signals fromsaid second pair of sensors are representative of the direction andextent of departures of said element from said centered position.

3. A stabilized magnetic levitation system according to claim 1, furtherdefined by the compensating flux generating means comprising a firstpair of diametrically opposed arcuate conductors each extendingsubstantially 180 coaxially disposed with respect to said magneticlevitation field in outwardly spaced circumscribing relation to saidelement, a second pair of diametrically opposed arcuate conductors 'eachextending substantially 180 coaxially disposed with respect to saidmagnetic levitation field in outwardly spaced circumscribing relation tosaid element at positions circumfcrentially spaced from said first pairof conductors by 90, and servo means coupled to said position sensingmeans for driving currents through said first and second pairs ofconductors with magnitudes and directions in accordance with saidsignals to thereby generate said magnetic compensating flux.

4. A stabilized magnetic levitation system according to claim 2, furtherdefined by the compensating fiux generating means comprising first andsecond pairs of diametrically opposed arcuate conductors each extendingsubstantially 180 coaxially disposed with respect to said magneticlevitation field in outwardly spaced circumscribing relation to saidelement, said second pair of conductors circumferentially spaced fromsaid first pair of conductors by 90, first servo means coupled to saidfirst pair of sensors for comparing the signals therefrom and developinga first position signal proportional to the algebraic differencetherebetween, and second servo means coupled to said second pair ofsensors for comparing the signals therefrom and developing a secondposition signal proportional to the algebraic difference therebetween,said first and second position signals being thereby representative ofthe direction and extent of departures of said element from saidcentered position, said first and second servo means respectivelycoupled to said first and second pairs of conductors to drive currentstherethrough in accordance with said first and second 8 position signalsand thereby generate said magnetic compensating fiux.

5. A stabilized magnetic levitation system according to claim 4, furtherdefined by said first servo means including a first bipolar differenceamplifier having input terminals connected to said first pair of sensorsand an output terminal, said amplifier generating said first positionsignal at the output terminal thereof, and a first controlled voltagesupply having an input terminal and a pair of sets of output terminals;said supply generating voltages at said sets of output terminals inaccordance with a signal at said input terminal thereof, said outputterminal of said difference amplifier connected to said input terminalof said supply, said pair of sets of output terminals of said supplyrespectively connected to opposite ends of said first pair ofconductors, said second servo means including a second bipolardifference amplifier having input terminals connected to said secondpair of sensors and an output terminal, said second amplifier generatingsaid second position signal at the output terminal thereof, and a secondcontrolled voltage supply having an input terminal and a pair of sets ofoutput terminals, said second supply generating voltages at said sets ofoutput terminals thereof in accordance with a signal at said inputterminal thereof, said output terminal of said second amplifierconnected to said input terminal of said second supply, said pair ofsets of output terminals of said second supply respectively connected toopposite ends of said second pair of conductors.

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